This invention relates in general to wrap spring operating mechanisms for use in clutch, brake, and combined clutch/brake assemblies to selectively connected first and second members together for concurrent movement. In particular, this invention relates to an improved structure for a wrap spring operating mechanism for use in such a clutch, brake, or combined clutch/brake assembly that limits the magnitude of acceleration or deceleration of the second member relative to the first member when the assembly is engaged.
Clutches are well known devices that are employed in wide variety of mechanical devices to selectively connect two members together for a desired purpose. A typical clutch structure includes a first member, a second member, and an operating mechanism for selectively connecting the first member to the second member. When the operating mechanism of the clutch is engaged, the first member is connected to the second member for concurrent movement. When the operating mechanism of the clutch is disengaged, the first member is disconnected from the second member to permit relative movement therebetween.
Clutches are frequently used in a driving mode, wherein the first member is connected to a source of rotational power and the second member is connected to a driven device. In these instances, when the operating mechanism of the clutch is engaged, the source of rotational power is connected through the clutch to rotatably drive the driven device. Conversely, when the operating mechanism of the clutch is disengaged, the source of rotational power is disconnected from the driven device to prevent such operation. Clutches are also frequently used in a braking mode, wherein the first member is connected to a stationary member and the second member is connected to a rotatable device. In these instances, when the operating mechanism of the clutch is engaged, the rotatable device is connected through the clutch to the stationary member so as to affirmatively brake the rotation thereof. Conversely, when the operating mechanism of the clutch is disengaged, the rotatable device is disconnected from the stationary member to allow such relative rotation. The above-described clutch and brake structures may be combined into a single clutch/brake assembly, wherein the second member is rotatably driven when the operating mechanism of the clutch/brake assembly is operated in a first mode and is affirmatively braked when the operating mechanism of the clutch/brake assembly is operated in a second mode.
One well known type of operating mechanism for clutch, brake, and combined clutch/brake assemblies is a wrap spring operating mechanism. A typical wrap spring operating mechanism includes a helical spring that is wrapped about adjacent cylindrical portions of the first and second members. The wrap spring has a relaxed inner diameter that is slightly smaller than the outer diameter of the portions of the first and second members upon which it is wrapped. Thus, when the first member is rotated in a first direction, the spring wraps down upon and frictionally engages the first and second members such that the second member is rotatably driven by the first member. When the first member is rotated in a second direction, the spring expands out of frictional engagement with the first and second members such that the second member is not rotatably driven by the first member. Wrap spring operating mechanisms can be used both for selectively driving a second member to be rotatably driven by a source of rotational power and for selectively braking a second member to prevent such rotation, as described above.
To control the engagement and disengagement of the wrap spring clutch efficiently, a control member may be provided for selectively controlling the expansion and contraction of the wrap spring (and, therefore, the operation of the wrap spring clutch assembly) in response to an external actuator. Typically, one end of the helical spring is secured to the first member for rotation therewith, while the other end of the spring is secured to the control member. The external actuator is provided for selectively engaging the control member to either permit or prevent rotation thereof relative to the first member. In this manner, the control member can be operated to either contract or expand the spring and, therefore, operate the wrap spring clutch assembly as desired.
In one known embodiment, the control member is an annular control collar having one or more outwardly-extending stops provided on the outer surface thereof that can be selectively engaged by a pivotable actuator. When the pivotable actuator is moved into engagement with one of the stops on the control collar, rotational movement of the control collar is prevented. When this occurs, the wrap spring is either expanded or contracted, thereby either releasing or engaging the second member for rotation with the first member. It will be appreciated that this type of operating mechanism permits engagement and disengagement of the assembly only at discrete rotational positions of the first and second members. In another known embodiment, the control member is an armature that is selectively movable into frictional engagement with the second member for rotation therewith. Typically, an electromagnet is provided for effecting movement of the armature into such frictional engagement. When the electromagnet is energized, the armature is frictionally engaged with the second member for rotation therewith, thereby causing the wrap spring to connect the second member for rotation with the first member. It can be seen that this type of operating mechanism permits engagement and disengagement of the assembly at any rotational position of the first and second members. Thus, wrap spring operated assemblies of this general type are commonly referred to as random start clutches when used in the above-described driving mode or random stop clutches when used in the above-described braking mode.
One of the significant advantages of wrap spring operated assemblies is that the time duration required to contract the wrap spring about the first and second members to effect engagement is very small, typically about three milliseconds or less. Unlike conventional friction-operated clutches, the engagement time duration for wrap spring operated clutches is essentially constant regardless of rotational speed or load placed thereupon. This advantage can, in some instances, present a problem when the clutch is operated at relatively high speeds or when a relatively large load is placed upon the clutch. In these instances, an undesirably large impact load may be placed upon the driven device when the clutch is engaged. This is because the driven device is very quickly accelerated (when the clutch is used to rotatably drive the second member) or decelerated (when the clutch is used to affirmatively brake the second member) when the wrap spring operating mechanism is engaged. This rapid acceleration or deceleration can result in an undesirably large impact load being applied to the driven device or other portions of the system, especially when the clutch is operated at relatively high speeds or when a relatively large load is placed upon the clutch. These impact loads can result in premature wear or damage. Thus, it would be desirable to provide an improved structure for a wrap spring operating mechanism for use in a random start/stop clutch, brake, or combined clutch/brake assembly that limits the rate of acceleration or deceleration of the second member relative to the first member when the assembly is engaged.